In batch type ion implanters a number of wafers are mounted on a cooled platen forming part of a spinning implant disc and exposed to the ion beam for doping semi-conductor wafers. This arrangement allows increased throughput by using a high current ion beam while maintaining the temperature of the wafers within the acceptable range.
Many efforts have been made to improve ion implantation technique by elimination of contamination and cross-contamination.
One source of contamination is sputtering of accumulated ion species from wafer holders exposed to the ion beam onto the wafer. These species function as cross-contaminants of previous implantations.
Another source of contamination is particles falling on the wafer during wafer processing. Most particles are produced by improper clamping of the wafer. The particles disposed on the wafer surface before the implantation process screen the implantation. The particles which fall on the wafer after implantation cause contamination of the following processed wafer.
Early devices for retaining wafers during ion implantation utilized either peripheral clamping mechanisms, for example, devices with a clamp ring engaging the front surface of the wafer or centrifugal clamping apparatus to hold the wafer on the cooled platen.
Along with the desire to prevent particulate formation and cross contamination in both types of clamping, the wafers must be protected from physical damage during the process. To avoid breakage of the wafers, they should be accurately positioned at the disc before clamping them thereon.
Peripheral clamping devices such as a clamp ring obstruct part of the front surface of the wafer and are susceptible to particle generation effects. Because of build-up of sputtered material on the clamp ring, the ring can be a source of particulate contamination. During the implantation process, the clamping ring is exposed to the ion beam and becomes a source of sputtered contamination from the clamping ting to the surface of the wafer. Moreover, the shadowed portion of the wafer is not exposed to the implantation process.
Centrifugal clamping techniques eliminate the need for clamping members that contact the wafer surface, and reduce the amount of wafer holder exposed to the ion beam above the wafer's plane. A centrifugal clamping arrangement provides the whole surface of the wafer for ion treatment, but specific mechanically complicated disc structure is required for producing sufficient centrifugal force for clamping.
One attempt to provide better utilization of the wafer surface and reduce both particle generation and sputter contamination effects was disclosed in U.S. Pat. No. 4,817,556 "Apparatus for Retaining Wafers" assigned to the assignee of the present invention. This mechanism is located primarily on the backside of the platen and uses edge handling. A flexible collet with a number of fingers around the periphery of the wafer retains the wafer by contacting its edge protruding above the surface of the platen. The platen has a plurality of grooves around its periphery, and each groove accommodates a finger. The outside portion of the collet is molded into and supported by an elastomer ring so as to allow the collet to flex, moving the fingers outward so that the wafer can be loaded onto the platen. The multiple grooves in the platen expose the flexible collet and its elastomer supporting ring to the ion beam. When the collet is actuated the material deposited by the ion beam, as well as the wear particles generated in the mechanism, are ejected and can deposit on the front surface of the wafer.
A multi-axis robotic system which is used in U.S. Pat. No. 4,817,556 for loading wafers onto the platens of the implant disc creates a certain amount of positioning error, particularly when operating inside a large vacuum chamber where perfect alignment is difficult to achieve. The rather limited movement of the fingers provided by the flexing of the collet makes this design sensitive to these positioning errors. Any misclamped wafer could be broken during the implant process causing loss of valuable material and expensive downtime to clean the implanter. Therefore, reliable operation of the wafer retaining mechanism could be provided by including wafer misclamp detection on systems, so that misclamped wafers could be reclamped.
An attempt to solve the positioning problem is described in U.S. Pat. No. 4,744,713 "Misalignment Sensor for a Wafer Feeder Assembly" detects wafer position to prevent misclamping of the wafer by sensing a collimated light beam reflected off the surface of the wafer. To implement the design a light source and a light sensor have to be placed on a supporting structure inside the vacuum chamber of the ion implanter far enough from the surface of the wafer to allow the disc transition from the wafer loading position to the implant position. To ensure reliable detection of misclamped wafers, a precise alignment of the implant disc and the components of the sensing system has to be maintained. Though the device effectively provides security against misalignment in the mounting of the wafer and avoids possible physical damage to the wafer, the clamping of the wafer is maintained with a peripheral clamping mechanism. This wafer clamping system includes a retractable spring loaded clamp with two arms which engage the wafer when loaded, wherein, in operation part of the wafer is overshadowed by the arms. The arms being exposed to the ion beam accumulate species which become cross-contaminants in a following implantation process.